Light meter having reflecting means to receive light from two opposite directions



May 30, 1950 F. PERLIN 2,509,366

LIGHT METER HAVING REFLECTING MEANS T0 RECEIVE LIGHT FROM TWO PPSITE DIRECTIONS Filed Aug. 25, 1947 IN VEN TOR. .FED PEQL/N, BY @2 (ff/Agg',

Trae/Vey,

Patented May 30, 1950 UNITED STATES orifice asoasse e DIRECTIONS Fredle'i'lin; Ios Angeles; Calif; Applicatio August 1947', snat No. riant 11 dans. (ci. 85;;23)

This invention relates generally to" improved apparatus and method forV use in regulating photographic exposure whereby the brightness range of the object or plurality oobjec'ts in a scene to be photographed maybe effectively com; pressed if they exceed the lnearportion of the H-D or Hurter-Driield curve, so as tofall on the linear portion of the said curve" whereby the photographic reproduction will have brightness variations correctly relateduto one another so as to correspond tothe brightness variations of the object or plurality of ob'jects in the scene photo` graphed. e e e l I i e The I-l-D curve referredto herein, as is wellA known in the art, is a graph of dnsity as a func-` tion of exposure, where the ordinates correspond to density and are on a linear scale, and the abscissas correspond `to exposure and are on` a logarithmic scale. `"lli'eI-I-D curveyisp'ositively curvedY in the region of small exposures and densites, is negatively curved in the region` of large exposures and densities and is virtually linear therebetween. Further information maybe olotained iniany textbook on photography and sensitometry. 1. e e e e i This invention is particularly applicable to the field of color photography, since, as is Awelllnown in the art, the emulsions used in` chromatic or color photography are much more critical than those used in achromatic or ordinary black andV white photography. e M Y y At the present timegin color photography it is necessary, generally speaking, `to use lath lightf ing, that is, lighting directed ontheobjects to be photographed from a point closely adjacent to the camera so that the light rays are virtually parallel to the camera and illuminate the `object or plurality of objects to be photographed in a manner casting virtually no shadows. This is necessary, because any Very considerable variation in den-l sity of the photographic reproduction will cause a serious color error. e e e In certain cases where `a plurality of objects has very different brightness values, even such nat lighting is'not satisfactory, since the allow-` able range of photographic density variations Vis exceeded, thus resulting in considerable color error, Furthermore, in both color photography and black and white photography an imperfect reproduction results where the brightness variations of the scene photographed are of such range as to be broader than the linear portion of the H-D curve of the negative andpositive' emulsions used in making the photographic'rproductionl This results in" a serious" loss o'f tonev'alue in the reproduction. For exairiplie',` ordinary aehrofmatic photography afscene includin'ga grey "tone, as viewed by the eyes, the gr iie win orten A apparsas a black in tile pho'tog pine' reprenne; non withyirtuauycompleteren 'of tone value andwith an improper `i'elatioiishi'p tlfireof` to the fest of trie various `ierivsities of th piitographi reproduction. A, er1-fumatore, suppose that m uref event that a; riuiriber of pie uresare being taken of various objects of different bright"` ss,` itis dill sconto stammte what-exposeren usemppottgraphing the various objects so .that the photo'i graphic reproductions thereof will each haveeth 15,1 prdper dannati/aine Vin r'eiatmn te` each' other.

For example, if a relatively bright .object is phOto-l graphed with an exposure determined by a priorarp type of exposure mteaani 4oneri `a second photographers mate of areiatiyay dani object with an exposure determined by said exposure metenthe densityof the photograph reproduc tions thereof will not be rlate'd'to each other as theorignal objects were. i

y 'one type of prioif art espesura maar has `gaierally comprisedanelectricallyresponsive metier ifoniiii i0@ riifziofseiisiiive `Celi iii ,they belmiary-layer type adapted "to, be place'dga short disi tance in iront or and facing Various portionsof` a scene to be photographed. For example, light." from both the brightest,portions of the scene and the darken ,Piiiiiiii Qfih ssriegify be spafeieiy en@ iiiiiividiiaiiyimeasvred by similari siiposure meter. Thentgthe exposure ot the whole scene may be determined so that the brightest or` darkest portions will fall at any selectedyp'oint on the H-D Currie, However, generally speaking, the brightness range is of such breadth as to lex; oecd,` the linear portionof the I-l-D curve and either' iiie biisiiisi rfiisn 01 iii@ darkest Portion or both will haveto be placed on `non- 1 inear curved portions o f the H-D curvethus resulting in an imperfect photographicreproduction of the" SCeii rhiogfaiiiid This isavsfy `gf'eii dis: advantage in achromatimphotography and is a fatal defect in color photography. i e f v ,e `It will readily be understood that themedian density of a" wholeiseries of photographic reprOf ductions Will all be properlywrelatedoneto the other if the incident light illuminating each scene isexactly the same. For this purpose a photo? graphic exposure meter of thetype hereinbefore mentioned may be' placedin frontwoi each object inH a scene to .be rnasured and directed toward the illuminating iriideht. iisiii`if0 determining the` general lightintensityonthe scene but not the" light reflected on the scene. However, this scene, in selected proportions, and varying the incident light on the various objects in the scene until the combined measured light equals a selected value, whereby to eifectively compress the brightness range of the photographic reproduction to the linear portion of its H-D curve.

It can readily be understood that through the use of the apparatus of the present invention, all

the prior art disadvantages mentioned hereinabove are avoided since the photographie reproductions will be on the linear portions of the H-D curve and thus correct density relationships will be maintained at al1 times. This is highly desirable in achromatic photography and absolutely essential in successful color photography.

With the above points in mind, it is an object of this invention to provide new and improved apparatus and method for faithfully reproducing the brightness variations of various objects in a scene photographed.

It is a further object of this invention to provide a photoelectric exposure meter adapted to combine and integrate selected proportions of light reflected from, and light illuminating, an object in a scene to be photographed.

It is a further object of this invention to provide apparatus and method in making photographic reproductions faithfully reproducing the brightness variations of the various objects in a scene to be photographed by effectively compressing the brightness range of the photographic reproduction to the linear portion of its H-D curve.

Other and allied objects will become apparent to those skilled in the art upon a careful examination, study, and analysis of the illustrations, specifications, and appended claims.

To facilitate understanding, reference will be had to the following illustrative drawings, in which:

Fig. 1V is a partial vertical section through one illustrative embodiment of this invention.

VFigl 2 is a fragmentary vertical section of a slightly modified form of the invention shown in Fig. 1.

Fig. 3 is a fragmentary vertical section through another illustrative embodiment of this invention.

Fig. 4 is a perspective View of another illustrative embodiment of the invention.

More specifically, referring to Fig. 1, a curved, closed, opaque hood I is mounted on a housing 2 adapted to contain electrically responsive means 3, which may be current responsive or voltage responsive, and provided with a visually observable, movable indicator 4, which is arranged to cooperate with visually observable indicia, indicated at 5, carried by a dial face 6. The indicator d, indicia 5 and dial face 6 are all visually observable through a transparent window 'I in the rear side of the housing 2, v

Horizontally positioned at the junction of the opaque hood I and the housing 2 is a photo-sensitive means 8, which may preferably, but not necessarily, be a photographic cell of the boundary layer type capable of vgenerating voltages,

7 5 jacentthe objectto be photographed, with the when light impinges the boundary layer, such as', for instance, a cuprous oxide cell or a selenium cell. The light-responsive, photo-voltaic means 8 is connected to the electrically responsive means 3 (microammeter or millivoltmeter) in a well-known manner. The opaque hood I is provided with a front translucent portion I3 and a rear, translucent portion 9 which, in the examples shown, are aligned. These translucent portions may be of any suitable material. For example, opal glass may be utilized and, in fact, the entire hood may be formed of opal glass with all portions thereof other than the portions I3 and 9 coated with an opaque material, such as indicated generally at IIl for rendering same opaque, if desired.' Various other constructions may also be employed.

Angularly positioned within the hood I is a partiallytransparent mirror II having a reflecting surface I2 angularly facing the forward translucent portionY I3 and also angularly facing the upper surface of the photo-sensitive means 8, whereby light reflected from an object in the scene to be photographed may enter the hood I through the transparent portion I3 and be reflected from the mirror I I onto the upper surface of the photo-sensitive means 8, as indicated by the arrows. The rear translucent portion 9 receives a portion of the incident light illuminating the object, since the entire unit is held in front of the object anda short distance therefrom, with window I3 facing the object being photographed and 9 facing the camera. The portion of the incident light entering the translucent portion Q and passing through the mirror I I and reflected from the inside wall of the translucent portion I3 backwardly onto the reflecting side I2 of the mirror II is thence downwardly directed onto the upper surface of the photo-sensitive means also, as indicated by the arrows.

It can be seen that since only a relatively small portion of the incident light will be reflected from the inside wall of the translucent portion I3 backwardly onto the reflecting surface I2 of the mirror II, a relatively small portion of the incident light will impinge the photosensitive means B in comparison with a relatively larger portion of the light reflected from the object and the reflecting Ysurface I2 `of the mirror II onto the upper surface of the photo-sensitive means 8. The effective proportion of the incidentlight and the reflected light which actually impinges the photo-sensitive means 8 may be adjusted in a number of ways. For example, the translucency of the portions I3 and 9 may be varied, if desired. The size of the translucent portions may be varied, if desired, by suitable masking or otherwise, and Various other means may be employed for proportioning the incident and reflected light received by the photo-sensitive means 8. This adjustment will vary, depending upon the photographic emulsions used, as more fully explained hereinafter.

The combined reilectedand incident light received by the photo-sensitive means 8 is transvlated by the photo-sensitive means 8 into corresponding electrical variations which, in turn, are translated by the electrically responsive means 3 into corresponding positions of the indicator 4 with respect to the dial face 5, which may be visually observed through the transparent window 'I.

The operation of the device may be described as follows.` The apparatus is placed closely adfront translucent Sortioir` I3 positioned a short distaz'ieetrierfroi so as to riot liaietod wide a field of r'eflectirigA lig-ht through the trans'-` li-icleiit portion I 3l intjth hood I. Then the indicator is obser/'ed Prt/lonely, the eter has been al-ibratedto correspond with the enu1-' s io'iisfbeig usedso that a" certain selected readV` ing' of tll indicator With respect to the' in-` dicia 5A will 'place the brihtnessf rang f the objeo'tsto belpl'iotographed on the desired lineal' portion of the- Pf-D curve.

non if die reaoiirigobserved from an object ila scietoiloe photographed isother than the sel'el'itedfeeding, the lightignthat-object may be` v'ariolur'itil a reading withina desired range is" dbserved this done for various objects inthe scene", when the-photographic"reproduction is iade, all the objects will be ifi their correct tonal relatiosliip's, since the Whole `brightness range lies on the linear' portion of the H-'D Curve. This applies to pifirioiial objects in the scei toi be photographed; Shadows may cause alesser read-ingtobe observed onvthe' rneterfsayv for' eiialple, half the Selected reading for' the rest of the Scne; This? effectively' results in eX- patrolling` the brightness range for said shadows, thus abhiev'ing the desiredy contrast. Thisl last arrangement is purely optional and may be employe'dwleie such effects are" desired.

Fig,Y 2 illustrates ai slightly' modified forrn of the apparatus shown iii-'Fig` l. Insteadof using partially transparent mirror, suon as mirror lisliowri in Fig. l, ape'iforated, burnished plate III iiiay be utilized;l Ascanr'eadily be seen,ithis acts in ainanner similar' to the mirror II shown in rig. igsiiioe tno maior portion of tno iight reflected noni an object to `te photographed and passing through the translucentportion I3 will be reected from the burnished plate If4 onto the upper surface of the photo-sensitive means E. A* smallportio'ii of the incident light passing through the rear translucent portion 9 will pass through the central aperture I5 iii-the burnished plate lliA and be reflected froin the inside Wall f the front translucent portion I3 back onto the burnished plate HI and thence downward onto the upper surfaceof the photo-'sensitive means 8. The balance of thedevice operates in a manner similar to that shown in Fig. 1 and described hereinabove.

Fig. 3 illustrates another embodiment of the invention in illustrative formi. In this case the ptical nieans employed comprises two mirror' surfaces IE" and I'I angularly arranged between the front translucent portion I3", and the rear translucent portion 9", and arranged to `clown-- wardly direct light received through-- said trans-V lucent portions onto the upper surface of the photoesensitiv'e nieans 8". The mirror surfaces I G and IT Amaybe individualmirrors-or they may be formed integrally if desired and may [bei nioiintd in any suitable nianner. The proportioriing f` the incident and reected light re ceived by the photocell 8 may be arranged by masking the translucent portions, mirror sui'-l faces, or `portions of the photocell. In other respects, this embdinientof my invention is sim# ilar to that hereinbeforedescribed.

It should, of course, be understood that any suitable reflecting means may be employed to reflect? the light rays" entering the device through" the trlaris'iuceiitl portions and direct them onto the photosensitive surface.

Fig. 4 mostro/es another embodiment of the invention' functioning accordance with the iid 6 sainel principles employed in the previous enibod` iiieiits of this inventionl;` althoghiof somewhat diffrent donst'ruction. `Inthislcaser any standard photoelectric exposure meter, such as that' in-l dicateulgnerally at I8", maybe employed. The exposure* dieter I8"s provided with# Viewv anglerestricting means'wnicn, in the example shown, comprises al shieldA I9 but' which may take the form of a narrow angle lens or a plurality ot parallel plates or a grillage or other type or View angle=liiniting means.y This" is for the purpose of limiting the" angle or view of light received by the exposure. meter I8 to an area a selected amount 'greater than' a White or light-grey card 2U carried atafixeddistance from the exposure iriete'r` IB on an extensible support, such as 2|, which niay comprise telescopng tubes and rods placing cardl 20fa`t a predetermined distance from the meter. Trie operation of this arrangement is' asionows: The exposure meter la is planea in iront of an object to be photographed', withthe? standard white c'ard 2U placed in contact with, or immediately in front of, the object. In" Fig. l that portion of the object embraced by the field or View of meter yI8 i's represented by the dash line square 22. It can' readily beseen that theV incident light illuhiriatiri'g the object 22 also illuminates the standard White card 2U, liflfii'ch is rel'ectd in'to the exposure meter to` gen-ier with' iigni reflected from predetermined aeadf the' object' 22 ibetwenthead and. dash lii lfiit.V Itla` Betsee that since the Standard white c ard 25 is ofA a certain' Seltd sz'e in cilnpariswith` a field 'of View f- `the photo# electrical"iiposureihter, thlight reflected from saidA card 20 corresponds" to the incident light` andV is in a proportion or ratio to the light reflected from the object 22, corresponding to their respective* areas. Thus, in eiect, exactly the saine result is obtained as that obtained with tlieprviuseiiibodiriintsof this invention hereildefe described. It should behted that light reflected from the standard Cardi 2i] will be con-i .sidered and rf'rred to` hereinafter as incident light, andlightrei'iectedtfrom the object 22 will,

as nei'einbeforo indicated, be referred to as refieoted ngnt.

actua-l practice, the transparencyof mirror I2, the trasllicency ofV ports 9 and i3, the size f port I5; or the masks used over the cell, are So arranged that a reading" of 2.5 to 3.5 as high ifsfobtained for light entering through the port Ihas'the reading obtained for light of saine intensity entering through port 9.

Numerous modifications and variations of this invention are possible within the scope and spirit, therefore', and will occur to those skilled in the art, and all such are intended to be included and oniprehended herein. l

In order to adapt the device for indoor and outdoor use, for use in high intensity and low key illumination, etc.. each level being repre# sented by a separate row of indicia readable by rheans of a single index 4, suitable masks may be interposed over the entire area of the lightsensitive cell 8. Such mask may consist of a inem-ber' rotatable on an axis parallel to the apex of Iirrors I6 and I1 (Fig. 3) and between s'uch apexand the cell, so that when the member ijs perpendicular to the cell plane, all of the light impinges on the cell, and when the member is at a predetermined angle to the `cell plane, only a portion of the light iinpinges the cell, thereby permitting accurate readings even when higher i intensity illumination is" used. The structure of the device may be varied within wide limits. Various types of photo-sensitive means other than the boundary layer type of cell may be used.

The examples described and illustrated herein are exemplary and illustrative only and are not intended to limit the scope of this invention, which is to be interpreted in the light of the appended claims only.

Iclaim:

1. A photoelectric exposure meter for use in photography, comprising: an opaque vhood provided with two aligned, translucent portions in opposite sides thereof, one of said portions being adapted to diifusedly admit light reflected from an object to be photographed and the other of said portions being adapted to diffusedly admit incident light illuminating the object; a partially transparent mirror angularly positioned between said translucent portions with the reflective side facing the light reflected from the object; a single photo-sensitive cell of the photo-voltaic type positioned out of alignment with said translucent portions and in a position to receive light reflected from the object and the reflective side of the mirror, and thatportion of the incident light which passes through the partially transparent mirror and is reflected from the opposite inside wall of the hood back to the reective side of the mirror, whereby the electrical output of the photo-sensitive cell will correspond to the sum of the light reflected from the object and actually impinging the photo-sensitive cell and that portion of the object illuminating incident light actually impinging the photo-sensitive cell; and electrically-responsive indicating means for visually indicating the response of the photosensitive cell.

2. A photoelectric exposure meter for use in photographpy, comprising: an opaque hood provided with two aligned, translucent portions in opposite sides thereof, one of said portions being adapted to admit light reflected from an object to be photographed and the other of said portions being adapted to admit incident light illuminating the object; a substantially V-shaped mirror means positioned between said translucent portions with one reflective side thereof angularly facing the translucent portion admitting incident light and the other reflective side thereof angularly facing the translucent portion admitting light reflected from the object; a single photo-sensitive cell positioned to receive reiiected light and incident light from the mirror, whereby the electrical output of the photo-sensitive cell will correspond to the sum of the light reflected from the object and actually impinging the photo-sensitive cell and that portion of the incident light actually impinging the photo-sensitive cell; and electrically-responsive indicating means for visually indicating the response of the photo-sensitive cell.

3. A photoelectric exposure meter for use in photography, comprising: an opaque hood provided with two aligned, translucent portions in opposite sides thereof, one of said portions being adapted to diffusedly admit light reflected fromv an object to be photographed and the other of said portions being adapted to difusedly admit incident light illuminating the object; a partially transparent mirror angularly positioned between said translucent portions with the reflective side angularly facing the light reflected fromthe object; a single photo-sensitive cell positioned out of alignment with said translucent portions and adapted to receive light reflected from the object and the reflective side of the mirror and that portion of the incident light which passes through the partially transparent mirror and is reflected from the opposite inside wall of the hood back to the reflective side of the mirror, whereby the electrical output of the photo-sensitive cell is an integrated response to both incident and reflected light; said translucent portions of the hood, said mirror and said photocell being so arranged as to effectively proportion the incident light impinging the photocell and the reflected light impinging the photocell to a. selected ratio whereby the density range of a photographic reproduction of the scene will lie on the linear portion of the H-D curve when the combined light impinging the photocell is within a predetermined range of values; and electrically-responsive indicating means for visually indicating the response of the photo-sensitive cell.

4. In an exposure meter for use in photography, the combination of a photo-sensitive cell; means for directing light reflected from an object to be photographed onto the photo-sensitive cell, said means including a reector and a light receiving means positioned on the meter to receive light from one side thereof; means operative at the same time such reflected light is being directed onto the cell for also receiving and transmitting to the same cell incident light falling from any direction upon the aspect of the object to be photographed, said last named means including a reflector and a light receiving means positioned on the meter and facing its other side; and means for visually indicating the response of said cell to the incident light falling upon the object to be photographed as modified by the light reected therefrom, both in the same sense.

5. In a light meter for use in photography: a photo-electric cell; light receiving and reflecting means for directing light approaching the meter from one side thereof onto the photo-sensitive surface of said cell; light receiving and reecting means for directing light approaching the meter from the opposite side of the meter onto the photo-sensitive surface of said cell; whereby upon placement of the meter closely in front of an object to be photographed with one of said light receiving means facing the object and the other facing away from the object, the response of the electric cell is a measure of the incident light which falls upon the aspect of the object to be photographed as modied by the light reected therefrom electrically responsive means for visually indicating the response of the photo-electric cell; and means connecting said electrically responsive means with the cell in such a manner that all light reaching the photo-sensitive surface of the cell causes said means to respond in the same sense.

6. A light meter for use in photography, comprising: an opaque hood having a pair of translucent windows in opposite sides thereof; a single photo-sensitive cell extending across the bottom of the hood with its receiving surface normal t0 the longitudinal axis of the hood; means for reecting light transmitted by both of said translucent windows onto substantially the entire surface of the sensitive cell; and means for visually indicating the cumulative response of the photoject and the other facing away from the object. the indication obtained is a measure of the incident light which falls upon the aspect of the object to be photographed as modified by the light reflected therefrom.

7. A light meter for use in photography, comprising: an opaque hood having a pair of translucent windows in opposite sides thereof, one of said windows being adapted to diifusedly admit light reflected from an object to be photographed and the other of said windows being adapted to diffusedly admit incident light from all angles at its side of the hood; a single photo-sensitive cell extending across the bottom of the hood with its receiving surface normal to the longitudinal axis of the hood; reflector means angularly positioned within the hood between said translucent windows and adapted to reflect the light diffusedly transmitted by them onto the photosensitive cell; and means for visually indicating the cumulative response of the photo-sensitive cell to the light admitted through both translucent windows.

8. A light meter for use in photography, comprising: a casing having opposite sides; light diffusing windows in said opposite sides, said windows being substantially parallel to the longitudinal axis of the casing; a photo-sensitive element in the casing with its receiving surface substantially normal to said longitudinal axis of the casing; light reflecting means within the casing for directing light entering one of the windows onto said photo-sensitive surface; light reflecting means within the casing for directing light entering the other window onto the photosensitive surface; whereby, upon placement of the meter in front of an object to be photographed with one of said windows facing the object, incident light falling upon the aspect of the object to be photographed and light reflected from the object are both impinged upon the photo-sensitive surface to activate it; and means for visually indicating the integrated response of said photo-sensitive surface to said two types of light.

9. A light meter for use in photography, comprising: a photoelectric cell having a photo-sensitive surface lying generally in a plane; light reflecting means above the photo-sensitive surface facing two opposite directions parallel with the general plane of the photo-sensitive surface for simultaneously directing onto said surface light which approaches the meter from said two opposite directions; and means for visually indieating the cumulative response of the photo-sensitive cell to the light approaching the meter from said two opposite directions.

10. The light meter dened in claim 9 further characterized by the fact that a part ofthe said light reflecting means is disposed at an angle of substantially 45 to the general plane of the photo-sensitive surface and faces said surface.

11. A light meter for use in photography, comprising: a casing having opposite sides; a photosensitive element having a light receiving surface within the casing; a light admitting and directing system within the casing defining an optical path, said system comprising windows in said opposite sides of the casing substantially parallel to the longitudinal axis of the casing, light diffusing means adjacent to the windows for diffusing light entering the same, light reflecting means for directing light entering one of said windows onto the light receiving surface of the photosensitive element and light reflecting means for directing light entering the other window onto the light receiving surface of the photosensitive element, said light receiving surface being substantially normal to that portion of the optical path of the system across which the light receiving surface extends; whereby upon placement of the meter in front of an object to be photographed with one of said windows facing the object, incident light falling upon the object to be photographed and light reflected from the object are both impinged upon the light receiving surface of the photo-sensitive element to activate the same; and means for visually indicating the integrated response of said photosensitive element to said two types of light.

FRED PERLIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,990,396 Ballard Feb. 5, 1935 2,023,159 Wright Dec. 3, 1935 2,198,971 Neufeld Apr. 30, 1940 2,294,876 Walker Sept. 1, 1942 2,378,433 Riszdorfer June 19, 1945 2,389,617 Freund Nov. 27, 1945 OTHER REFERENCES Ser. No. 357,992, Riszdorfer (A. P. C.), published May 4, 1943. 

